A method and an apparatus of the kind mentioned at the outset are disclosed in U.S. patent specification 5,250,239 of the same applicant, the contents of which is incorporated herein by way of reference.
From this prior art it is well known to provide a method and an apparatus for controlling the movements or displacements of two components of a plastic material injection molding machine which may be displaced relative to each other. The components are in particular a mold or a mold portion and/or a gripper or, speaking in more general terms, a handling system. Conventional molds may comprise one stationary mold portion and one movable mold portion, however, within so-called stack molds, there are three distinct mold portions, namely, a center platen and two further mold portions located on opposite sides of the center platen. Irrespective of the type of mold the mold portions execute a closing and an opening movement for allowing an injection cycle to take place in the closed state of the mold whereas the injected and finished plastic material articles may be unloaded from the load in its open state.
For unloading finished plastic material articles or for inserting items into the mold cavities prior to injecting plastic material, it is known in the art to use handling systems or robots having a gripper. The gripper enters into the gap between the open mold portions and is retracted therefrom prior to the closing of the mold. In many cases such handling systems are autonomous installations having autonomous control systems which are coupled to the control of the plastic material injection molding machine. However, in some instances the handling system or the gripper is a structural component of the plastic material injection molding machine itself.
For the sake of a simplified description of the invention, the subsequent description starts from the latter assumption without, however, thereby limiting the scope of the invention.
Within the art of plastic material injection molding machines one has the general problem that the gripper must enter between the mold portions in their open state and must be retracted therefrom for unloading a finished article and/or for inserting a specific item into the still empty mold cavity. This means that the trajectories of the movable gripper on the one hand side and the movable mold portion on the other hand side spatially overlap at least partially. As a consequence, the movement or displacement of these components must be precisely synchronized in order to avoid collisions therebetween.
The safest option to achieve this object would be to initiate the movement of one of the components only at a moment in time when the respective other component has left the trajectory of the first movable component. However, this has the disadvantage that the cycle time of the injection molding machine becomes very long due to the necessary delay times. The term "cycle time" is to be understood as the period of time needed by the plastic material injection molding machine for a complete loading, injecting and unloading cycle until the next cycle may be initiated.
U.S. patent specification 5,250,239 mentioned above suggests to initiate the movement of the first component already at a moment in time when the second movable component has not yet entirely left the trajectory of the first component. For that purpose both components are provided with two sensors each, one of which measuring the position and, by determining a period of time, also the velocity of the component at that particular position. The other sensor is connected to a control unit generating an enabling signal for the movement of the first component when the second component has passed the position of its second sensor with a predetermined velocity.
By measuring the period of time needed by the second component for moving through the distance between the two sensors associated to it, one has a guarantee that the enabling signal is generated only when the second component moves with a velocity corresponding to normal operation at a predetermined position. Due to the inertial masses of the components involved one may assume in this case that even when a failure occurs, the second component will at least travel through such a distance that no collision occurs.
Optimizing the cycle time in such a plastic material injection molding machine while concurrently avoiding collisions even in the case of an operational failure highly depends on the position of the second sensor and, accordingly, from the moment in time when the enabling signal is generated. If the second sensor is located such that the enabling signal for the first component is generated too early, a collision due to a failure of the normal operation may not be entirely excluded.
The worst case that may occur insofar would be when the drive for the second component fails at a moment in time in which the enabling signal for the first component is generated. In that case the first component will be started while the second component may not leave the trajectory of the first component with the speed that it would assume under normal operational conditions.
If, on the other hand side, the trigger point for the enabling signal is set too late, the cycle time for the injection molding machine is unnecessarily extended.
U.S. patent specification 5,250,239 is silent about the possibilities or methods for determining and setting an optimum trigger point for generating an enabling signal.
German patent specification 41 10 948 discloses a method for unloading injected plastic material articles from an injection molding machine. According to that method the movement of a first component shall be initiated at the earliest possible moment in time for increasing the cycle frequency and, hence, shorten the cycle time of the injection molding machine. For doing so, the motional sequences of the two components are each compared with a displacement vs. time function stored in the control unit in order to readjust the motional sequence in case of an unallowed deviation from the predetermined motional sequence. An emergency shutdown is also provided. However, in view of the above-mentioned optimization, this prior art only discloses in general terms that a collision shall be avoided by stopping movements in case of an emergency. The specification is silent about how to determine the trigger point for generating the enabling signal in an optimized way.
It is, therefore, an object underlying the present invention to provide a method and an apparatus of the kind specified at the outset, making a plastic material injection molding machine cycle time as short as possible on the one hand side while, on the other hand side, definitely avoiding a collision of the moved components even under worst case assumptions. It is, further, an object of the invention to provide an appropriate plastic material injection molding machine.